Methods and systems for thermal energy storage and recovery

ABSTRACT

Thermal energy storage and recovery methods and systems are provided herein, which utilize a thermal energy storage vessel. The vessel comprises a packed bed of chemically inert particulates exhibiting high thermal conductivity. A gaseous heat transfer fluid (e.g., steam) is fed to the vessel, whereby at least a portion of the fluid condenses on the particulates and transfers latent heat to the particulates. During a heat recovery step, a heat recovery fluid (e.g., air) is fed to the vessel, whereby sensible heat transfers from the particulates to the heat recovery fluid. The warmed heat recovery fluid may then be used to provide required heat for a variety of applications.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention is generally directed to methods and systems forthermal energy storage and recovery.

Description of the Prior Art

Due to growing energy production from intermittent energy sources suchas solar and wind, and due to the increasing skewedness in energy demandpatterns, there have been several studies for understanding thermalenergy storage systems. Recently, there have been numerous studies wherethe thermal behavior of the storage systems was studied during storageor recovery processes. These studies have been performed for solid aswell as liquid storage media. However, in all of these studies the heatcarrier or heat transfer fluid was in a single phase (i.e., in a gas orliquid state for all scenarios). Systems utilizing single phase heattransfer fluids have experienced unacceptably slow thermal energy gain,and thus the total energy storage achieved by such systems has beensimilarly unacceptable. Additionally, these systems have generallyexhibited less predictable heat dispersion as the single phase fluidpasses through the energy storage vessel non-uniformly, resulting invariable warm and cool spots with less efficient energy storage.Therefore, improved thermal energy storage methods and systems areneeded.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, there is provided a methodof storing thermal energy contained within a condensable fluid for lateruse. The method comprises feeding a gaseous heat transfer fluid into avessel comprising a packed bed of inert particulates. The method furthercomprises contacting the gaseous heat transfer fluid with the inertparticulates and condensing at least a portion of the heat transferfluid on the inert particulates. The contacting and condensing steptransfers at least a portion of the latent heat contained within thegaseous heat transfer fluid to the particulates. The method alsocomprises storing the portion of the latent heat within the particulatesfor a period of time until at least a portion of the stored latent heatcan be recovered from the particulates.

In another embodiment, there is provided a thermal energy storage systemcomprising an evaporator adapted for vaporizing a fluid stream, a vesselcomprising at least one packed bed of inert particulates and having atleast one fluid inlet and at least one fluid outlet, and a conduitconfigured to direct the vaporized fluid stream from the evaporator tothe at least one fluid inlet. The at least one fluid outlet isconfigured to remove a condensate of the vaporized fluid stream from thevessel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of one embodiment of the presentinvention, wherein solid lines represent fluid flow during normal systemoperation and dashed lines represent fluid flow during heat recoveryoperation.

FIG. 2 is a schematic drawing of another embodiment of the presentinvention, wherein solid lines represent fluid flow during normal systemoperation and dashed lines represent fluid flow during heat recoveryoperation.

FIG. 3 is a is a schematic drawing of another embodiment of the presentinvention, wherein solid lines represent fluid flow during energystorage operation and dashed lines represent fluid flow during heatrecovery operation.

FIG. 4 is a schematic drawing of an experimental setup of a packed bedheat sink.

FIG. 5 is a series of images showing the experimental packed bed vessel(top left), as well as X-ray images of the internal fluid flow (top) andthermal images of the external surface of the vessel (bottom) during theenergy storage process.

FIG. 6 is a temperature plot showing the results for the slow injectionsteam experiments.

FIG. 7 is a temperature plot showing the results for the fast injectionsteam experiments.

FIG. 8 is a temperature plot showing the results for the air injectionexperiments.

FIG. 9 is a schematic drawing of drawing of one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Inventive methods and systems for storing thermal energy are providedherein. The inventive methods and systems utilize a thermal energystorage vessel comprising a packed bed of inert particulates. Theparticulates are capable of storing thermal energy transferred from ahot gaseous heat transfer fluid. Thermal energy can then be recoveredfrom the particulates by passing a heat recovery fluid through thepacked bed. The warmed heat recovery fluid can then be utilized in anumber of applications requiring a source of heat.

One embodiment of the present invention is shown schematically inFIG. 1. As shown in FIG. 1, an evaporator 110, thermal energy storagevessel 120, and heat exchange unit 130 are provided in a system. Thesecomponents are described in greater detail below. During normaloperation, a heat transfer fluid stream 112 is fed to evaporator 110,which provides gaseous heat transfer fluid streams 114 a and 114 b.Stream 114 b is fed to heat exchange unit 130 to provide necessarythermal energy for normal system operation. For example, in certainembodiments, heat exchange unit 130 may form a part of a buildingheating system or an absorber refrigeration system. The spent heattransfer fluid exits unit 130 as outlet stream 132. Stream 114 a is fedto vessel 120, wherein thermal energy from the heat transfer fluid istransferred to solid particulates contained within vessel 120. Asexplained in greater detail below, at least a portion of the gaseousheat transfer fluid is condensed within vessel 120 thereby transferringlatent heat to the particulates contained therein. The condensed fluidexits vessel 120 via outlet stream 122. This heat is stored by theparticulates until it is desired to recover that energy for use withinheat exchange unit 130, such as when evaporator 110 is offline. Duringheat recovery operation, the output of gaseous heat transfer fluid fromevaporator 110 is reduced or ceased. In order to provide the necessarythermal energy to heat exchange unit 130, a heat recovery fluid stream126 is fed to vessel 120, wherein thermal energy stored in the solidparticulates is transferred to the heat recovery fluid, thereby forminga warmed heat recovery fluid stream 128. Warmed heat recovery fluidstream 128 is then fed to heat exchange unit 130 thereby ensuring acontinuous supply of heat energy to the process or system of which unit130 is a part.

Another embodiment of the present invention is shown in FIG. 2. Theembodiment of FIG. 2 operates similarly to the embodiment of FIG. 1,except for the arrangement of process equipment. During normaloperation, a heat transfer fluid stream 212 is fed to evaporator 210,which generates gaseous heat transfer fluid stream 214. Stream 214 isfed to thermal energy storage vessel 220, wherein thermal energy fromthe heat transfer fluid is transferred to solid particulates containedwithin vessel 220 resulting in the at least partial condensation ofstream 214. Although a certain amount of thermal energy is transferredto the particulates and stored within vessel 220, a substantial amountof useful thermal energy may remain in the heat transfer fluid and bepassed through vessel 220. The heat transfer fluid exits vessel 220 andis fed to heat exchange unit 230 via stream 222 to provide necessarythermal energy for normal system operation. The spent fluid exits heatexchange unit via stream 232. During heat recovery operation (i.e., whenevaporator 210 is offline), the necessary thermal energy required byheat exchange unit 230 is provided by feeding a heat recovery fluidstream 226 to vessel 220. Similar to the process of FIG. 1, the thermalenergy stored in the solid particulates is transferred to the heatrecovery fluid, thereby forming a warmed heat recovery fluid stream 228.Warmed heat recovery fluid stream 228 is then fed to heat exchange unit230.

Yet another embodiment of the present invention is shown in FIG. 3. Theembodiment of FIG. 3 operates similarly to the embodiment of FIG. 2,except that the arrangement of the thermal energy storage vessel and theheat exchange unit are reversed. During normal operation, a heattransfer fluid stream 312 is fed to evaporator 310, which generatesgaseous heat transfer fluid stream 314. Stream 314 is fed to heatexchange unit 330 to provide necessary thermal energy for normal systemoperation. Although a certain amount of thermal energy may be removedfrom the fluid in heat exchange unit 330, a substantial amount of usefulthermal energy may remain in the heat transfer fluid, particularly ifthe heat load of unit 330 is relatively low. The heat transfer fluid isthen passed through heat exchange unit 330 and exits via stream 332.Stream 332 is fed to thermal energy storage vessel 320, wherein thermalenergy from the heat transfer fluid is transferred to the particulatesand stored within vessel 320, resulting in at least partial condensationof the gaseous heat transfer fluid, which exits vessel 320 via outletstream 322. During heat recovery operation, the necessary thermal energyrequired by heat exchange unit 330 is provided by feeding a heatrecovery fluid stream 326 to vessel 320, transferring thermal energystored in the solid particulates to the heat recovery fluid (therebyforming a warmed heat recovery fluid stream 328), and feeding warmedheat recovery fluid stream 328 to heat exchange unit 330. The spent heatrecovery fluid exits unit 330 via stream 334.

Although certain preferred embodiments are described above, it should beunderstood that these embodiments are not intended to be limiting andthat other process equipment and arrangements may be utilized within thescope of the present invention. That is, one of ordinary skill in theart would understand that the thermal energy storage vessel (or multiplethermal energy storage vessels) and heat exchange unit (or multiple heatexchange units) may be provided in parallel or in series in a variety ofcombinations. It is also understood that the system equipment describedhere is operably connected using various conduits for transporting theworking fluids.

The heat transfer fluid may be any of a number of fluids capable ofconducting and transferring thermal energy through the system. Asdescribed in more detail below, preferred heat transfer fluids arecapable of being vaporized in the evaporator and at least partiallycondensed when contacted with the particulates in the thermal energystorage vessel in order to take advantage of latent heat energy releasedduring condensation. Preferred heat transfer fluids will have a normalboiling point above about 75° C. In certain preferred embodiments, theheat transfer fluid has a normal boiling point temperature from about75° C. to about 200° C., preferably from about 85° C. to about 150° C.,more preferably from about 95° C. to about 125° C., and most preferablyabout 100° C. In certain embodiments, the heat transfer fluid maycomprise refrigerants, such as R134a or others that undergo phasechange, organic volatile solvents including glycol-based fluids, such asethylene glycol and propylene glycol, and steam, with steam beingparticularly preferred. Depending upon system configuration (e.g.,thermal storage occurring upstream or downstream of the heat load) thesteam may be in the form of superheated steam or saturated steam.However, it is preferred that steam is supplied to the thermal energystorage vessel as saturated steam.

The heat recovery fluid may similarly be any of a number of fluidscapable of conducting and transferring thermal energy through thesystem. In certain embodiments, the heat recovery fluid may be the sameas or different from the heat transfer fluid. However, in particularlypreferred embodiments, the heat recovery fluid is different from theheat transfer fluid. In certain aspects of the present invention, it isimportant to utilize a heat recovery fluid that will not undergo a phasechange when brought into contact with the heated inert particulates.Thus, the heat transferred to the heat recovery fluid is sensible heat.In certain embodiments, the heat recovery fluid is selected such that itwill enter and leave the vessel entirely in the gaseous state, butshould not be taken as excluding the use of liquid heat recovery fluids.In particularly preferred embodiments, the heat recovery fluid is air.However, it is also within the scope of the present invention that theheat recovery fluid be any other heat transfer fluid that can recoverenergy in the form of sensible heat, latent heat, or a combination ofboth, such as water, DOWTHERM™, heating oils, refrigerants, and others.

The evaporator is capable of supplying sufficient energy to the heattransfer fluid so as to vaporize the fluid. For example, in preferredembodiments, the evaporator is a renewable steam generator, meaning thatthe energy required to vaporize the fluid is obtained from a renewablesource. Renewable steam generators include, for example, solar steamgenerators, biomass steam generators, and wind-powered steam generators.

However, it is understood that non-renewable evaporators may also beused if desired or required by a particular application. For example,the heat transfer fluid may be heated using a product stream from anexothermic reaction process or a heated flue gas stream. The heatexchange unit is capable of supplying heat to any number of processes.For example, in particularly preferred embodiments, the heat exchangeunit may be a component in an indoor heating system, absorptionrefrigeration system, or any other domestic or industrial process heat,power generation, or cooling application.

The thermal energy storage vessel may be constructed in a variety ofgeometries, using a variety of materials. The vessel generally comprisesan outer housing, one or more fluid inlets, and one or more fluidoutlets. In certain embodiments, one or more of the fluid inlets mayalso be utilized as a fluid outlet, so long as the vessel is configuredsuch that the heat transfer fluid may be passed through the vesselduring normal operation and that the heat recovery fluid may be passedthrough the vessel during heat recovery operation. The vessel furthercomprises at least one packed bed containing the inert particulates. Incertain embodiments, the packed bed comprises a cylindrical chamber andthe inert particulates comprise a plurality of solid particles. Theparticles may be a variety of shapes, including, for example, spherical,cylindrical, elliptical, irregularly shaped, or combinations thereof. Inpreferred embodiment, the particles have an average diameter of fromabout 0.1 mm to about 8 mm, preferably from about 0.5 mm to about 6 mm,and more preferably from about 1 mm to 3 mm. Regardless of the particleshape, as used herein the particle diameter refers to the lineardistance across the particle as taken across its largest dimension. Inparticularly preferred embodiments, the particles are sphericalparticles having an average diameter of less than 3 mm. The particlesmay comprise a variety of materials. However, the materials selectedshould generally be chemically inert and exhibit high thermalconductivity. In certain embodiments, the particles comprise at leastone of alumina, graphite, silica, quartz, ceramic, or rock (e.g., peagravel). In particularly preferred embodiments, the particles comprisealumina and/or pea gravel. The vessel may also be insulated using avariety of methods and materials known in order to minimize heat loss tothe surrounding environment during energy storage. Exemplary thermalenergy storage vessels are described in U.S. Application Publication No.2014/0202157 and U.S. Application Publication No. 2014/0299306, both ofwhich are incorporated by reference in their entireties, herein.

Embodiments of the present invention demonstrate improved thermal energytransfer, storage, and recovery over prior art methods and systems dueto the unique heat transfer phenomena taking place within the thermalenergy storage vessel (and particularly within the packed bed portion ofthe vessel) during normal system operation and subsequent heat recoveryoperation. In contrast to prior art methods which used single-phase heattransfer fluids to transfer thermal energy to storage systems, thepresent invention preferably uses a condensable fluid that exists inboth the vapor and liquid phase (e.g., saturated steam) during transportthrough the packed bed. It has been discovered that the use of suchcondensable fluids as the heat transfer fluid during thermal the initialenergy transfer phase provides faster and more predictable heat transferconditions. Without being bound by any theory, it is believed that theimproved heat transfer behavior is due to the combination of two typesof thermal energy transfer occurring within the vessel during thisoperation. First, when the vaporized heat transfer fluid condenses onthe packed bed particulates, latent heat is released by the fluid andtransferred to the particulates. The term “latent heat” is generally thechange in internal energy experienced by a body or thermodynamic systemwith no change in temperature. Thus, the latent heat transfer aspect isdue to the phase change (condensation) without a change of fluidtemperature. This transfer of latent heat provides a rapid and morepredictable transfer of a greater quantity of heat to the solidparticulates as compared to a transfer of mere sensible heat from theheat transfer fluid. Second, when the hot vapor or liquid contacts thecooled particulates, sensible heat is transferred from the fluid to theparticulates due to the difference in temperature. The term “sensibleheat” is generally the change in internal energy experienced by a bodyor thermodynamic system as measured by the temperature change. Thus, thesensible heat transfer aspect is due to the temperature differencebetween the fluid and the particulates.

Based on the above-described principles, embodiments of the presentinvention comprise contacting a gaseous heat transfer fluid with inertparticulates within the packed bed. During the initial energy transferoperation, the particulates are generally cooler than the fluid. Due tothis temperature difference, sensible heat is transferred from the fluidto the particulates, and thus the fluid temperature decreases. When thefluid temperature reaches the boiling point temperature within thepacked bed, at least a portion of the fluid condenses on theparticulates, and latent heat is transferred from the fluid to theparticulates. While the fluid condensation does not change the fluidtemperature, the temperature of the particulates and the packed bedincreases significantly during this step. After condensation occurs,some additional sensible heat transfer may continue to occur if there isa temperature difference between the fluid and the particulates.Advantageously, it has been discovered that the use of a condensableheat transfer fluid (e.g., steam) in the packed bed configurations ofthe present invention allows for improved cross-sectional uniformity asthe fluid is fed through the packed bed and energy is transferred to theparticulates. Thus, an observable and predictable thermal front isexhibited as the heat transfer fluid is fed into the packed bed and heatis being stored. Accordingly, during as heat transfer fluid is being fedto the energy storage vessel, a sufficiently steep temperature gradientis maintained along the flow direction, which discourages exergy lossesdue to thermal dispersion. The rate at which the gaseous heat transferfluid is fed to the packed bed will influence that rate at which energyis transferred to the particulates. At higher feed rates, faster energytransfer is typically observed. However, the feed rate is limited by thefact that the heat transfer fluid should have a sufficient residencetime within the packed bed such that the fluid can condense and releaselatent heat. Generally, the residence time (and related flow rate)should be selected based on other design parameters (e.g., particulatesize, particulate material, and type of heat transfer fluid) such thatthe particulates have corresponding Biot numbers less than 0.1. The heattransferred to the packed bed particulates is then stored within theparticulates for a period of time until recovery is desired (e.g.,during times of reduced heated vapor generation).

During heat recovery operation of the system, the heat recovery fluid isfed into the thermal energy storage vessel and passed through the packedbed containing the heated particulates. The heat recovery fluid shouldhave a cooler temperature than the heated particulates but should alsobe fed to the vessel in a single, preferably gaseous, phase. In contrastto the heat transfer fluid, the heat recovery fluid preferably passesthrough the packed bed in a single phase. For example, a gaseous heatrecovery fluid is fed into the vessel and the fluid remains in thegaseous phase throughout the heat recovery operation in the vessel withlittle or no phase change. As the recovery fluid does not experience achange in phase, there is no energy transfer due to latent heat. Rather,thermal energy is transferred to the heat recovery fluid only in theform of sensible heat. This has the advantage of providing a longerduration of heat supply, as the particulates do not lose the storedthermal energy as quickly as they obtained it. Therefore, inparticularly preferred embodiments, the heat recovery operation occursover a greater period of time than the initial heat transfer operation.

An exemplary energy storage and recovery system is shown in FIG. 9, andits operation utilizing steam as the heat transfer fluid, in accordancewith one embodiment of the present invention, is described below. Theembodiment described herein uses water as the heat recovery fluid inorder to recover steam, but it should be understood that other heattransfer fluids and heat recovery fluids can also be used within thescope of the present invention. During energy storage operation, valve21 opens to allow steam from an evaporator (not shown) to flow throughfluid inlet 22 and into vessel 20. As the steam contacts theparticulates 29 within vessel 20, the steam condenses, therebytransferring latent heat energy to the particulates 29. The condensedsteam (water) then exits vessel 20 through fluid outlet 24 and valve 25,and the heat is stored in the packed bed of particulates 29 until energyrecovery is desired. Preferably, vessel 20 is insulated or otherwiseconfigured to minimize or eliminate heat loss through the outer surfaceof the vessel during heat storage. During energy recovery operation,valves 21 and 25 are closed, and water is introduced into vessel 20through water inlet 26 and valve 27. When the water contacts the hotparticulates 29, heat is transferred from the particulates 29 to thewater, thereby vaporizing the water into steam. The steam then exitsvessel 20 through fluid outlet 28 and valve 23 as recovered thermalenergy.

Example

The following example sets forth radially and azimuthally symmetricsteam condensation heat transfer experiments using a packed bed ofalumina particles. It is to be understood, however, that these examplesare provided by way of illustration and nothing therein should be takenas a limitation upon the overall scope of the invention.

Introduction

Conventional thermal energy storage systems have been largelyineffective due to complete mixing in the storage medium as the heattransfer fluid is introduced. When heat transfer fluid is introduced viamixed flow, the temperature of the energy storage medium increases;however, the temperature profile throughout the energy storage mediumand storage vessel at any particular point in this process is relativelyuniform. Such mixed flow methods require that in order to bring thetemperature in entire bed from cold to hot will require the discharge ofwarm to hot fluid during the entire storage process (i.e., wasting largeamounts of energy). As described in more detail below, a more preferredscenario is to achieve a plug flow (i.e. no mixing downstream) whichenables steep temperature gradient. This is because bringing the entirebed from cold to hot temperature in such a scenario would not requireany discharge of warm or hot fluid, and thus any fluid leaving thestorage vessel during the storage process is always cold.

Another physical process which limits the ability to achieve plug flowis temperature diffusion within the storage medium. This occurs, forexample, when heat transfers within the storage medium ahead of the heattransfer fluid. Although some prior art storage solutions have been ableto achieve some gradient in the temperature profile of the storagemedium during storage operation, this gradient is relatively gradualfrom inlet to outlet. Accordingly, these solutions have been unable toachieve the steep temperature profiles associated with plug flow due tothe temperature diffusion within the storage medium. Such solutions havebeen shown to achieve thermal energy storage efficiencies not more than50-60%.

Design Objectives

Previous studies assumed that a packed bed configuration enables theuniform flow and temperature distribution in the radial direction (i.e.,in the direction normal to the flow). However, due to wall heat losses,the temperature of the wall and the bed were different from each other.Thus, accurate analysis required simultaneous wall temperaturemeasurements. One of the primary objectives is to understand the axialdispersion of the temperature front upon steam injection, temperaturemust be measured at various locations in the axial direction withoutinterfering significantly with the system behavior. This experimentfocuses on steam condensation at atmospheric pressure, which simplifiesthe design of the vessel and fittings. One of the difficulties inunderstanding the steam condensate process is associated with theuncertainties in the flow conditions and thickness of the two phase(liquid-vapor) zone inside the packed bed. Moreover, effective thermalconductivity inside the bed is strongly dependent upon instantaneousliquid hold-up and interface location. Therefore, the condensate flowrate was measured or estimated throughout the experiment to correlatewith the temperature response.

Experimental Setup

All experiments were performed in a cylindrical quartz tube randomlypacked with spherical particles. The size of the cylindrical vessel was6 inches (15.24 cm) tall and 2.5 inches (6.35 cm) diameter. The cylinderdiameter was the only size limitation because of the standard ceramicflanges used to seal the ends. The diameter of the spherical aluminaparticles were 3-mm in order to provide the largest available ratiobetween the sphere diameters and the cylinder tube diameter. Having aratio of tube to particle diameter greater than 20 also allowed for theplug-flow assumption or radially uniform dispersion of the thermalfront.

The spherical alumina particles were procured from Saint-Gobain NorProunder the commercial name Denstone® 99. These commercially availableparticles were chosen because they allowed for uniform isotropic heatingand have been previously tested for chemical inertness and robustthermo-mechanical behavior with steam. Alumina particles have high heatcapacity, high thermal conductivity, and chemical inertness, whichallows the rapid localized equilibration of thermal energy between thefluid phase and solid phase. Alumina is also non-degradable, allowing itto last a relatively long time and to remain stable through multipleheating and cooling cycles. Large heat transfer surface area wasachieved due to considerably smaller particle or packing size ascompared to the overall bed dimensions. This made the thermal frontpropagation more predictable and along the flow direction. Withsaturated steam as the heat transfer fluid, the rate of heat rejectionis much faster during the condensation process. Therefore, if the mediahas sluggish response to absorb heat, this will lead to very complexenergy balance in three phases. Therefore, using alumina particlesachieves the crucial design objective of performing reproducible thermalbehavior tests.

The quartz tube was sealed with ceramic flanges at both ends to providelowest possible energy dispersion effects from the boundaries. Thequartz tube was chosen because it provided for visual inspection ofmovement of the liquid-vapor interface and the known constant emissivityvalue of quartz material in temperature range of 25° C.-100° C. allowedfor easy measurement of wall temperatures with an IR camera.

The measurement setup to attain the temperature values along the outsidewall of the heat sink vessel was a forward looking infrared (FLIR)camera. The packed bed temperatures were measured with an Omega®multi-point thermocouple tube to record the temperature at six axiallocations in the bed. The multi-point thermocouple was positioned asclose to the center of the bed as possible using a fitting screwed intothe top flange of the vessel. Each thermocouple was numbered respectiveto its position from the inlet of the test chamber.

In-house steam supply was used in the experiments. Before starting theexperiments, it was ensured that steam supply pressure and flow did notchange for the duration of the individual experiments. This was done byallowing the steam to condense in a cylindrical flask with cold waterand monitoring the change in the level with time during steam flow. Inaddition, during the actual experiments of steam injection in the packedbed condensate, flow was monitored to take into account theuncertainties, if any. Steam was supplied from the top of the testchamber after passing through a pressure regulator holding the backpressure constant for all experiments. A globe valve was situated justbefore the entrance to the test vessel to allow control of steam afterthe supply valve was opened. This combination allowed the evaluation ofthe system's response to a step input of constant pressure steam. Thedownstream end of the test chamber was connected to a tube-in-tube heatexchanger where any remaining vapor was condensed. This extra stepallowed for the total mass of steam that passed through the chamber tobe collected and measured, enabling a value for the total amount ofenergy input into the system to be obtained.

A simplified schematic of the experimental setup is shown in FIG. 4,with vessel 10 comprising a packed bed of particulates 12, fluid inlet14, fluid outlet 16, and multi-point thermocouple 18 havingthermocouples (TC-1-TC-6).

Experimental Procedure

The experiments were performed in two sets. The first set of experimentswere the slow-injection tests, wherein the steam was slowly andgradually injected into the packed bed. In these experiments, athrottled steam supply was further regulated by a globe valve to nearlyatmospheric pressure before being injected into the top of test section.This throttled condition was confirmed by the fact that the maximumtemperature of the steam at the top of the cylinder did not risesignificantly above 100° C. Prior to each test run, the system wasflushed with dry cold air to ensure uniform temperature and no vaporcontent in the system. Steam was then continuously injected through thecylinder until such point as the thermographic camera registered thatthe wall of the cylinder had achieved steady state conditions.

The second set of experiments were the fast-injection tests. In theseexperiments, the globe valve was left completely open and flow injectionwas initiated with a butterfly gate valve. After the correct initialconditions were achieved (i.e., the alumina particles were both dry andat room temperature), and after setting the desired steam inletconditions, the butterfly gate valve was quickly opened to inject thesteam into the system. The steam was continuously injected into thesystem at the set pressure (i.e., atmospheric pressure) until thethermographic camera displayed a uniformly heated outer wall. Whilesubstantially all of the steam was condensed in the tube before beingdischarged in the slow injection tests, in the fast injectionexperiments the steam was allowed to flow through the packed bed andexit out the bottom into a discharge pipe.

The injection flow rate of the steam for each case was determined bycondensing the steam, collecting the condensate, and timing how long thevalve was open. Multiple experiments were run to ensure repeatabilityand consistent flow rate values for both the slow and fast injectioncases. The average condensate collection flow rates for the slow andfast were measured to be 1.25 cm³/s and 45 cm³/s. In the followingsection, instead of exact flow rates, discussion will be made using theterms slow and fast injection. For each experiment it was found thatuncertainty in the measurement of condensation collection flow rate iswithin 5% of the numbers stated above.

The condensate was collected and measured for the experiments using anair supply to remove all of the liquid before and after the run and abeaker to collect all of the condensation during and after a run. Thevelocity was assumed constant, and thus only flow rates were measuredfor experiments where the bed was completely filled. After observing theexperimental results trends for the slow-injection case, it was decidedto only fill the bed partially and measure the different condensing flowrates. After testing this, it was found that the velocity did change.Specifically, it was noticed that the velocity had a negative linearslope as the bed was being heated. Using this varying velocity data, theaccuracy of the models' solutions was improved, as presented in theresults sections. To compare the steam injection experiments to a singlephase heat transfer fluid, air was heated and sent through the randomlypacked bed. The flow rate of the air was controlled with an aircompressor and its outlet nozzle. The air was allowed to reach a steadystate temperature by bleeding out through a valve located just beforethe packed bed, and once it had reached the steady temperature, theinlet and bleed valves were slowly opened and closed, respectively. Thisallowed the pressure to remain relatively unchanged so that the steadystate air would enter the packed bed without any uncertainties. Thisallowed the experiments to be easily reproduced and led to accurate dataand results.

Results

Thermal Imaging.

To observe the uniform heat rejection process in the bed with radialsymmetry, thermal images of the cylindrical quartz walls were capturedwith frequency 6.25 fps. X-ray images of the interior of the vessel andthermal images along the wall of the packed bed for one of theexperiments are shown in FIG. 5. Without the solid media in the vessel,the steam would circulate through the containment and condense randomlyalong the walls. With the solid media present, the thermal frontpropagation images at different times shown in FIG. 5 reveal that thesteam condensation over the packed bed is radially uniform, indicatingplug flow. Therefore, qualitative and quantitative analyses using thedata obtained can be done in axial direction (1-D) i.e. direction ofsteam flow.

Vapor Flow Modeling.

Steam or vapor flow in hot porous media has been studied previouslyusing convection diffusion models. Radially symmetric time dependentone-dimensional convection-diffusion equation can be written as

$\begin{matrix}{{\frac{\partial T}{\partial t} + {\kappa \; v\frac{\partial T}{\partial x}}} = {\alpha \frac{\partial^{2}T}{\partial x^{2}}}} & (1) \\{\kappa = \frac{\left\{ {\rho \; C_{p}} \right\}_{f}}{\left\{ {\rho \; C_{p}} \right\}_{b + f}}} & (2)\end{matrix}$

where T is the local temperature of bed and fluid stream, x and t arethe axial dimension and time variable respectively, {ρC_(p)} is theenergy density per unit time, subscripts f and b denote fluid and bedrespectively, v is the fluid stream velocity and a is thermaldiffusivity of the bed. Although at inlet condition, the fluid stream issaturated steam and possesses latent heat, for simplified discussion inmathematical form this can be considered as specific heat spread overthe small temperature difference around the evaporation temperature.This convection-diffusion equation can be used to provide a simplifiedqualitative analysis of the vapor flow. The

$\kappa \; v\frac{\partial T}{\partial x}\mspace{14mu} {and}\mspace{14mu} \alpha \frac{\partial^{2}T}{\partial x^{2}}$

terms in Equation 1 are the advection and conduction terms,respectively. Assuming the temperature is zero at the exit or bottom ofthe bed, in the direction of the motion of the fluid, the temperaturegradient,

$\frac{\partial T}{\partial x},$

is negative, and the term

$\frac{\partial^{2}T}{\partial x^{2}}$

is positive. Therefore at any axial location in the bed, both of theseterms will lead to positive rate of change of temperature. In thefollowing subsections, the impact of slow and fast injections of steamon the temperature front progression at the points of measurement isdescribed.

Slow Steam Injection.

The characteristic thermal response of the packed bed system atdifferent times upon slow injection of steam is highlighted in thisdiscussion with explanation of results. In the case of slow injection,there are two thermal transport mechanisms—advection and conductionmodes at different spatial locations and different time frames. Near thesteam entry port, the temperature of the bed and fluid streams becamealmost equal to the steam inlet temperature or saturation temperature ina very short period of time. As steam supply was continuously available,irrespective of injection rate, the constant temperature conditions atthe inlet implies that the bed temperature at the top was alwaysmaintained at saturation temperature. This constant bed temperature atthe top (inlet) conducts heat from the top to bottom of the bed due tonon-negligible thermal conductivity of the alumina particles and waterin the bed (i.e., conduction mechanism). Simultaneously, the steaminjected into the bed was also carrying some amount of energy as itmoved in the bed (i.e., advection mechanism).

Due to the slow injection rate, an initial rise in the temperature ataxially farther locations was dominated by the conduction mechanism. Asthe steam or two phase mixture front, which is at a temperature near thesaturation temperature, reaches those regions located far away frominjection point, there was a sudden change in the temperature, as shownin FIG. 6, with temperature measurements at different times anddifferent locations. The rate of increase of temperature at differentlocations is divided into two distinct regimes with two distinct slopesfor the last four (3-6) locations. The initial regimes, which show lowerslope, are governed by conduction mechanism. The later regimes withhigher slope are governed by advection. These conclusions aresubstantiated with the observation that conduction dominated for longertimes for the locations farther from the injection point.

Fast Steam Injection.

Based on the explanations provided in the previous subsection, it wasexpected that the advection term would be much higher as compared toconduction term throughout the fast injection experiment. The higheradvection term implies that total amount of influx enthalpy carried bythe steam or two-phase mixture is much higher and thus, as the fluidstream moves through the bed it is equilibrating the bed to thesaturation temperature at almost constant rate at all spatial locations.Due to much higher rate of enthalpy injection in the bed due toadvection term, the effects of conduction were not expected have muchimpact on rate of temperature increase in the bed. The results in FIG.7, for fast injection, confirm this. In the slow steam injection case, ashort steep injection front was observed between the infiltrating steamand the run off condensation. While this phenomena is prominent in theslow injection case, fast injection showed simultaneous heat transferfrom both advection and diffusion throughout the bed. As the steam fronttraveled through the bed at much quicker pace, the conduction in the beddid not play a major role in transporting this heat to points in the bedfarther downstream.

The bottom thermocouple in the fast case was unusable because of howfast it responds to temperature change compared to the other fivethermocouples. The reason for this is not clear, so it was left out ofthe plots intentionally for the fast temperature cases. But for the slowcases, the temperature change at this location was slow enough that thiseffect was unnoticeable. In comparing slow injection and fast injection,the cases showed a considerable difference in the heat transfer andthermal front propagation through the packed bed. The most noticeabledifference was the reduced amount of time for the bed to reach peaktemperature throughout the bed in the fast injection case.

Air Injection.

Only one set of results are shown for the case wherein air was used asthe heat transfer fluid, as the difference in flow rate caused noprofound difference in the thermal front propagation in the packed bed.The slow injection air case is similar to the fast injection air casebecause no significant difference was demonstrated using theexperimental setup. However, one noticeable difference was that theslower case had a slightly higher temperature due to more heattransferred to the slower moving air. This allowed the air to reachhigher temperature before entering the packed bed. The top thermocouplewas bypassed by the air stream, and thus this reading was unreliable forassessing the temperature data and was not used. The packed bed at eachvertical position was at a lower temperature going from the inlet toexit of the vessel. This results in a more elongated thermal front ascompared to the case with slow steam injection. The experimental datafrom an air injection case is shown in FIG. 8.

Comparing the results of the experiments using air to those using steamas the heat transfer fluid, there are some very distinct observationswhich can be made. Due to the low energy density of air, it requires agreater amount of time to heat the packed bed. Even in the steady statesituation, the rate of heat loss through the bed walls were comparableto the rate of heat input injected in the bed. This led to temperaturegradients within the bed, even after a continuous steady state wasachieved, as can be seen in the time series plots of differentthermocouples. The plots show that lower end thermocouples remain atlower temperatures with steady state condition. In contrast, the highenergy density of steam (because of the latent heat) allows it tosaturate the packed bed to top temperature more quickly, as shown inFIGS. 6 and 7.

Conclusion

The IR camera images of experimental runs show that steam condenses withcross-sectional uniformity over the packed bed of spherical particles inthe directional plane normal to the steam flow, justifying the designbasis. The experiments were conducted with two modes of steaminjection—fast and slow mode. Thermal response of the bed was found tobe distinct in both of these cases. In case of slow injection mode, thetemporal behavior of the bed was found to be divided into two spatialzones, advection driven temperature rise in the bed near the steaminjection point and conduction driven temperature rise for the regionsfar from the injection point. As the slow moving steam front reached thefar zone, a steeper rise in the temperature was seen during later stagesof the experiment. Fast injection mode involved high enthalpy fluxpenetrating and equilibrating the bed quickly, and thus only advectiondriven temperature rise was observed at all spatial locations. Notably,as the steam condensed with cross-sectional uniformity and the latentheat was transferred to the particulates for storage, the packed beddesign was able to maintain a sufficiently steep temperature gradientalong the flow direction to discourage energy losses due to thermaldispersion. These tests show that efficiency of energy storage is closeto 99%. Advantageously, the system can achieve an overall efficiency ofenergy storage and recovery (i.e., round trip efficiency) between 95%and 98%, depending upon the type of recovery fluid and duration ofstorage.

As shown in the results above, different heat transfer fluids alsoexhibited distinct heat transfer behavior. The use a condensable fluid(steam) demonstrated much faster heat transfer to the packed bed due tothe latent heat transferred during condensation of the steam. Incontrast, the use of air as a heat transfer fluid exhibited much slowerheat transfer, as the heat transfer in that case was only achieved usingsensible heat transfer.

1. A method of storing thermal energy contained within a condensablefluid for later use comprising: feeding a gaseous heat transfer fluidinto a vessel comprising a packed bed of inert particulates; contactingsaid gaseous heat transfer fluid with said inert particulates andcondensing at least a portion of said heat transfer fluid on said inertparticulates, wherein said contacting and condensing step transfers atleast a portion of the latent heat contained within said gaseous heattransfer fluid to said particulates; and storing said portion of thelatent heat within said particulates for a period of time until at leasta portion of the stored latent heat can be recovered from saidparticulates.
 2. The method of claim 1, wherein said heat transfer fluidis steam.
 3. The method of claim 2, further comprising generating saidsteam using energy from a renewable steam generator.
 4. The method ofclaim 1, further comprising the step of recovering said portion of thestored latent heat from said particulates, said recovering stepcomprising: feeding a heat recovery fluid to said vessel; contactingsaid heat recovery fluid with said inert particulates and transferringthermal energy from said particulates to said heat recovery fluid in theform of sensible heat, thereby forming a warmed heat recovery fluid. 5.The method of claim 4, further comprising directing said warmed heatrecovery fluid to a heat exchange unit.
 6. The method of claim 5,wherein said heat exchange unit comprises a component of an absorptionrefrigeration system.
 7. The method of claim 5, wherein said heatexchange unit comprises a component of an indoor heating system.
 8. Themethod of claim 4, wherein said heat recovery fluid is air.
 9. Themethod of claim 4, wherein said recovering step occurs over a greaterperiod of time than said contacting and condensing step.
 10. A thermalenergy storage system comprising: an evaporator adapted for vaporizing afluid stream; a vessel comprising at least one packed bed of inertparticulates and having at least one fluid inlet and at least one fluidoutlet; and a conduit configured to direct the vaporized fluid streamfrom the evaporator to said at least one fluid inlet, said at least onefluid outlet configured to remove a condensate of the vaporized fluidstream from said vessel.
 11. The system of claim 10, wherein saidevaporator is a steam generator.
 12. The system of claim 11, whereinsaid steam generator is a renewable steam generator.
 13. The system ofclaim 10, wherein said vaporized fluid stream comprises steam.
 14. Thesystem of claim 10, further comprising: a heat exchange unit; and asecond conduit configured to direct a heat recovery fluid from said oneor more fluid outlets to said heat exchange unit.
 15. The system ofclaim 14, wherein said heat recovery fluid is air.
 16. The system ofclaim 14, wherein said heat exchange unit comprises a component of anabsorption refrigeration system.
 17. The system of claim 14, whereinsaid heat exchange unit comprises a component of an indoor heatingsystem.
 18. The system of claim 10, wherein said at least one packed bedof inert particulates comprises a cylindrical chamber and a plurality ofsolid, spherical particles having an average diameter of less than 3millimeters.
 19. The system of claim 18, wherein said particles have anaverage diameter of from about 1 mm to 3 mm.
 20. The system of claim 10,wherein said particulates comprise at least one of alumina, graphite,silica, quartz, ceramic, or rock.